1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device with a two-layer gate structure.
2. Description of the Related Art
In a conventional nonvolatile memory device with a two-layer gate structure, a single-layer thermal oxide film is used as an interlayer insulating film interposed between a floating gate electrode and a control gate electrode. On the other hand, in accordance with miniaturization of semiconductor elements, the thickness of the interlayer insulating film has been reduced. Where the interlayer insulating film is thin, a high electric field is applied to the interlayer insulating film while the memory device is charged. Consequently, there is a concern that the insulating film may be destroyed.
Under the circumstances, the use of an interlayer insulating film with a three-layer structure of an oxide film (first oxide film), a nitride film and an oxide film (hereinafter, referred to as ONO structure) has been proposed in order to increase a breakdown voltage of the interlayer insulating film. However, since the first oxide film formed directly on a floating electrode is a polycrystalline silicon thermal oxide film in which impurities are diffused at high concentration, the composition of the first oxide film is not uniform, and the quality of the film is not good. Thus, if the semiconductor device is reduced in size and the interlayer insulating film is made thinner, the charge retaining property of the device is degraded.
As stated above, in the prior art, even if the interlayer insulating film between the floating gate electrode and the control electrode has the three-layer structure, the charge retaining property of the device is degraded if the thickness of the insulating film is reduced due to the miniaturization of the semiconductor